July 23, 2002.
By Edward Chiao,
University of California.
Los Angeles researchers have helped to develop a "vision chip" which can help some blind patients see again.
The development is good news for one in four Americans over the age of 64, who suffer from a now curable form of blindness according to the Macular Degeneration Research Foundation.
In humans, normal vision starts when an image passes through the eyeball and into the retina at the back of the eye. The retina contains photoreceptors which break down the image and creates the corresponding neural stimulus, or signals, to the brain. In many blind patients, the retina degenerates and the photoreceptors no longer function, leading to blindness.
The vision chip works by replacing the function of the eye's retina, acting like a photoreceptor which processes images and sends the correct signals to the brain to form an image.
"In some people, the nerves still work but the retina is dead," said professor Warren Grundfest, chair of the biomedical engineering department at UCLA. "With this technology, electrodes are implanted in the back of the (patient's) eye, which is controlled by a very complex system."
The system was developed from research led by Second Sight, a four-year old company in Valencia, Calif.
It consists of a tiny video camera mounted onto a pair of glasses to capture images. These images are then processed by a tiny computer to prepare the visual information for the prosthetic chip implanted in the patient's eye.
The signals are then sent via radio frequencies to the retina chip, which receives the signals (acting like photoreceptors).
The chip finally sends corresponding electrical pulses through tiny electrodes connected to the chip, which stimulate neural responses to the brain, forming an image the blind patient sees.
The first such retina prosthesis was implanted into a patient five months ago at the Retina Institute at the University of Southern California Medical Center.
"We have been working with the patient to determine the best way to electrically stimulate his completely blind retina," said Dr. Robert Greenberg, president and CEO of Second Sight.
Currently, the vision chip in the patient's eye uses a simple two-dimensional 4x4 matrix to simulate vision. The patient can only see lines and shapes created by the low-resolution array.
"It allows for crude vision such as identifying orientation of objects and movements," Greenberg said.
But Greenberg said higher resolution arrays will help future patients see with more detail.
"A 1,000 electrode device (30x30 matrix) would provide a level of vision that most of us would consider adequate for everyday tasks," Greenberg said. "Even 100 electrodes would allow the identification of faces."
Greenberg is working with UCLA electrical engineering professor Jack Judy on increasing the resolution of the current low-resolution array.
Judy has experimented with micromachining the electrodes to increase their effective surface areas and reduce their size on the integrated circuit, allowing for more electrodes on the chip and better resolution.
"The whole concept is the miniaturization of things," Judy said. "I'm focusing on miniaturizing the electrodes on the retina chip, but there are limitations on their size."
In order to stimulate the neurons in the retina, the electrodes must be able to deliver a certain amount of current. Decreasing the size of the electrodes and keeping the current constant will increase the current density. If the current density increases beyond a threshold density, electrochemical reactions will occur.
"You will have certain problems (past the threshold current density)," Judy said. "It can cause hydrolysis (gas bubbles) or even corrode the metal."
Judy and Greenberg are still experimenting with several methods to increase the number of electrodes on the chip.
Greenberg is confident they will succeed.
"Our effort with Professor Judy is an important path that may allow such (high-resolution) arrays," Greenberg said. "I am sure we will achieve this goal by one of the methods being pursued."
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